PNIVEBSITY OF CALIFORNIA f PBLICATIOM8
COLLEGE OF AGRICULTURE
AGRICULTURAL EXPERIMENT STATION
BERKELEY, CALIFORNIA
SUN-DRYING AND DEHYDRATION
OF WALNUTS
BY
L. D. BATCHELOR and A. W. CHRISTIE
ASSISTED BY
E. H. GUTHIER and R. G. LaRUE
BULLETIN No. 376
March, 1924
UNIVERSITY OF CALIFORNIA PRINTING OFFICE
BERKELEY, CALIFORNIA
1924
Digitized by the Internet Archive
in 2012 with funding from
University of California, Davis Libraries
http://www.archive.org/details/sundryingdehydra376batc
SUN-DRYING AND DEHYDRATION OF WALNUTS*
BY
L. D. BATCHELOR and A. W. CHRISTIEt
ASSISTED BY
E. H. GUTHIER and R, G. LaRUE
CONTENTS page
Methods of Sun-Drying 4
Cost of Sun-Drying 6
The Use of Unheated Drying Houses 7
Early History of Walnut Dehydration 8
Recent Interest in Artificially Drying Walnuts 8
Extent of Dehydration During 1923 10
Advantages of Dehydration 10
Weather Protection 10
Theft Protection 10
Drying Efficiency 10
Labor Efficiency 11
Packing House Efficiency 11
Marketing Efficiency 11
Principles of Dehydration 11
Temperature Effects 13
Critical Temperature for Quality 13
Splitting of Nuts during Curing 15
Relation of Temperature to Drying Time 15
Starting Dehydration at Low Temperature 15
Intermittent Operation of Dehydraters 16
Relation of Humidity to Dehydration.... 17
Relation of Air Flow to Dehydration.. 17
Relation of Depth of Walnuts to Air Flow 18
Yields and Storage of Dehydrated Nuts 20
Types of Walnut Dehydraters 21
Natural Draft Dryers 21
Air Blast Dehydraters 21
Bin Dehydraters 22
Tray Dehydraters 24
Mechanical Movement of Nuts During Drying 25
Costs of Dehydration 25
* Paper No. 117, University of California, Graduate School of Tropical Agri-
culture and Citrus Experiment Station, in cooperation with Division of Viticulture
and Fruit Products.
t The authors wish to acknowledge the assistance rendered by the California
Walnut Growers' Association in donating Mr. LaRue's services as well as
appropriating a fund to further this work.
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
SUN-DRYING AND DEHYDRATION OF WALNUTS
Walnuts must be dried or "cured" before they can be marketed.
This is generally done by exposing them to the sun for several days.
The seasonal drying operations usually extend over six or eight weeks,
and in cool or moist weather may be very slow and imperfect.
Improper curing may reduce good nuts to ' ' culls ' ' or ' ' near-grades ' '
and depreciate their value 15 to 50 per cent and delay their delivery
at the packing house until the association's "second pool," which sells
at a discount of 10 or 15 per cent below the ' ' first pool. ' ' Non-associa-
tion growers are not directly affected by the closing of pools but it is
the usual practice of commercial buyers to accept late deliveries only
at a discount.
Losses of this kind caused by the periodical occurrence of damaging
weather in some of the walnut growing districts account for the present
interest in walnut dehydration.
As shown in detail on page 10, the advantages of dehydration in
promoting fine quality and early marketing because of rapid curing,
will be of great value in helping to profitably market the rapidly
increasing crops of California walnuts in competition with foreign
walnuts.
METHODS OF SUN-DRYING
Walnut drying, commonly referred to as curing, is for the purpose
of evaporating the excess moisture in the nuts which, if not removed,
would lead to spoilage and a lack of stability in weight. The method
as practiced by the pioneers of the industry was described by Joseph
Sexton* in 1888 :
The walnuts are picked up and put in sacks and barrels, so as to be easily
handled, and hauled to a sunny place to dry, and should be placed on ele-
vated platforms made of narrow boards, with spaces of one-fourth of an
inch between each board. The platform should be about 8 feet wide and 40
feet long, or as long as two men can handle a canvas to cover the beds, which
should be done every night the dew falls. The nuts should be stirred in these
beds once or twice each day, and with favorable weather they will dry sufficiently
in three days, and are ready for market.
Paper read before Ninth Fruit-Growers' Convention, 1888.
BULLETIN 376] SUN-DRYING AND DEHYDRATION OF WALNUTS 5
Most of the walnuts grown in California are now dried on trays
spread in the sunshine or in thoroughly ventilated drying-houses. The
practice is as follows : The nuts are spread on shallow slat-bottom trays
and exposed to sun and wind. If the air is especially dry and hot, the
trays should be spread out in the morning, and when the nuts are well
warmed they should be thoroughly stirred and the trays piled up. The
slower drying which goes on while the trays are in piles avoids the
splitting of poorly sealed nuts and their rejection as culls. Figure 1
shows a yard of trays being piled up after having been thoroughly
warmed by the morning sun. The trays should be so piled as to allow
ventilation between them.
Fig. 1. — Trays ready to pile up after having been thoroughly warmed
by morning sunshine.
The walnuts should not be exposed to showers, fogs, or even heavy
dews during the curing process. The alternate wetting and drying
of the shells which occur when the nuts are exposed to dew at night
and to sunshine during the day, cause many of the nuts to crack at the
apex. During the season of 1923, two trays of nuts were cured under
identical conditions except that one was covered from sun-down to
sun-rise while the other was exposed to the dew and fog. On the
uncovered tray 64 per cent of the nuts cracked, on the covered only
4 per cent.
The trays most commonly used are 3 by 6 feet, and 6 inches deep.
Extending the sides of the trays 12 inches beyond the ends makes
suitable handles. The bottoms are made of laths spaced one-half an
b UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
inch apart. The present price for such trays is approximately $2.00
each. Supports are needed for the trays to allow air circulation under
and through them while the nuts are drying. Waterproof covers are
used for the stacked trays to keep the nuts dry during rain or foggy
weather.
COST OF SUN-DRYING
The cost of curing walnuts on trays in the sun was made the subject
of a survey during the harvest period of 1923. Twenty-five walnut
growers were interviewed in the several walnut growing districts from
Irvine to Goleta. The average cost per ton amounted to $5.48, as shown
in table 1. This included the overhead charges of depreciation and
interest on capital invested in trays and the labor costs in the drying
yard from the time the nuts were received from the grove until they
were sacked ready for delivery to the packing house. The contract
price for performing this service in the districts visited averaged $7.35
a ton. According to figures gathered in 1920 by the Walnut Protective
League,* the cost of curing walnuts amounted to approximately $10.00
a ton.
TABLE 1
Cost of Sun-drying Walnuts*
Tray invesment
per ton
Labor
per ton
Overhead charge
per ton
Number
trays
required
Total
cost
Man
Hours
Cost at
40 cents
per hour
Interest
at
6 per cent
Depre-
ciation
at 5
per cent
Total cost
per ton
Maximum
Minimum
Average
12.00
2.66
7.78
$24.00
5.32
15.56
35.42
0.82
9.45
$14.17
.33
3.77
$1.44
.32
.93
$1.20
.27
.78
$16.81
.92
5.48
* Data from 25 growers producing from 7 to 135 tons, using standard 3x6
trays.
The survey summarized in table 1 indicates that the method of sun-
drying on trays requires 8 trays per ton of nuts cured during the
season. The average production of the groves surveyed was 1200
pounds per acre. With an average production of 1000 pounds per
acre a 10-acre grove would require 40 trays to properly cure the crop.
The investment in trays alone would approximate $80.00 per 10-acre
unit. In addition to this most growers have sheds in which to house
the trays when not in use and for the temporary storage of cured nuts.
* American Walnut Industry, by Walnut Protective League, Jan. 1921, p. 23.
Bulletin 376]
SUN-DRYING AND DEHYDRATION OF WALNUTS
The greatest single charge is for labor. According to the growers
interviewed, it requires an average of 4 man hours per day to care for
100 trays. Stated on a tonnage basis the average labor cost equalled
$3.77 per ton. It is often difficult to obtain the necessary labor as the
demand for labor during the walnut harvest season usually exceeds
the supply.
Despite certain disadvantages, sun-drying has always been used in
the curing of most of the crop. For small orchards it will likely
continue to be the most practicable method.
Fig.
-Sidwell drying
house, outer walls made of slats to give good
ventilation.
THE USE OF UNHEATED DRYING HOUSES
In curing large crops it will save labor to use a drying-house such
as the Sidwell dryer illustrated in figure 2. The drying houses are
usually built with outer walls of slats to give good ventilation. The
drying trays or floors are arranged one above another, 8 to 10 in
number. The nuts are elevated by conveyors to the top floor on which
they are spread in a layer about 6 inches deep. As drying progresses
the nuts are dumped mechanically from one floor to the floor below by
means of pivoted sections in the floors. The frequency with which the
nuts are dumped and consequently the number of days required for
drying will depend on the moisture content of the nuts and weather
conditions, usually 5 to 7 days in favorable weather.
8 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
In hot dry weather the nuts may pass through the drying house in
4 to 5 days. To realize their maximum efficiency such drying houses
should be exposed to the full effect of wind and sun without inter-
ference from adjacent buildings or trees.
When the nuts on the lowest floor are cured they are dumped into
hoppers from which a conveyor transfers them to one end of the drying
house for sacking. While the nuts pass along this conveyor, there is
an excellent opportunity for an initial culling before delivery to the
packing house.
The only advantages of such unheated drying houses are that the
nuts are protected from rains, and that the labor cost per ton of nuts
cured is less than for tray drying, but this saving is cancelled by the
greater overhead charges on the equipment. The saving in labor when
the walnut and other farm crop demands are at a peak may make the
use of a drying house more practicable than tray drying even though
it may be more costly.
The advantages of modern heated dryers for walnuts are obtainable
at no greater cost per ton than for unheated drying houses. (See
Advantages of Dehydration, p. 10.)
EARLY HISTORY OF WALNUT DEHYDRATION
The curing of walnuts by artificial heat apparently began in the
late eighties. Hon. Russel Heath* referred to the methods used in
artificially drying walnuts at that time, as follows :
In handling the nuts, I cure in dry-houses by artificial heat, heating sufficient
to evaporate the water and set the oil of the nut. When this is done, the nuts
will keep sweet for an indefinite time.
Apparently the early methods of artificially drying walnuts were
not entirely practicable, at least the industry was not converted to this
method of curing.
RECENT INTEREST IN ARTIFICIALLY DRYING WALNUTS
Aside from the artificial drying of walnuts by a few of the early
pioneers in the industry, there have been only a few cases of walnuts
being dried by this method until recently. Among the advocates of
this method of drying walnuts, the following may be mentioned :
The Corona del Mar Ranch at Goleta has dried walnuts in an old
prune dryer for the past 15 to 20 years. The nuts are held in trays
* Essay read by Hon. Eussel Heath before the Eleventh State Fruit Growers :
Convention 1889.
BULLETIN 376] SUN-DRYING AND DEHYDRATION OF WALNUTS 9
over a hot air flue, which is heated by a wood fire in a brick furnace.
The temperature maintained is based only on judgment and is regu-
lated by the draft of the furnace. The nuts are received in the dryer
after passing through the bleaching solution, and are then warmed for
a period of 10 to 12 hours.
In 1918 Mr. C. I. Crane of Santa Paula began the practice of drying
walnuts with heat from a furnace. The nuts were placed in a wooden
slat bottom bin with the heat entering at the bottom. In 1919 the
dryer was completely burned. In 1922 a new dryer was installed using
steam heating coils underneath a wooden slat bottom bin. Tempera-
tures of from 100° to 118° F. have been maintained, and the nuts dried
in from 12 to 48 hours according to their moisture content.
"During the summer of 1920, the San Joaquin Fruit Company*
installed a small crude-oil burner and a blower to force the heated air
to the drying shed. A bin was built which would hold ten tons of
walnuts, or a day's pick, and during bad weather the walnuts were
put in this bin and hot air at about 85° to 90° F., was forced in at the
bottom for about 18 hours, when the bin was emptied and the process
repeated with green nuts. In this manner the outside moisture was
driven off, and the curing process in the trays was made shorter. ' '
From this bin dryer the nuts were conducted on a belt to the floors
in the drying house.
During the summer of 1920, Mr. F. T. Mahoney built a walnut
dryer for Mr. E. C. Kimble of Saticoy. The nuts were poured into a
wooden bin with a slanting, slat bottom; hot air was forced into a
chamber below this bin, and the draft of hot air passed through the
nuts. In 1920 there were 26 tons of nuts dried in this bin dryer.
During the summer of 1921 the dryer on the Kimble ranch was
rebuilt, the bin being divided into four compartments. The fan for
delivering heated air to the bins was changed from a high speed blower
to a slower speed Multivane fan which delivered a greater volume of
air per horsepower consumed.
Conveying machinery was added to the Kimble dryer before the
harvest season of 1922. Three similar dryers were installed on other
ranches the same season. The addition of the conveying machinery for
elevating the nuts from a hopper into the bins was a labor-saving
device of pronounced value. The interest in this type of bin dryer
increased in 1923, when ten additional dryers of this type were con-
structed in Ventura County.
* From address by C. V. Newman, on Walnut Curing, read before Fourth
Annual Walnut Institute, Jan. 1922.
10 UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
The general interest in curing walnuts by artificial heat was greatly
stimulated during the spring of 1922, by an address by Mr. Carl
Newman before the Fourth Annual Walnut Institute, held at Santa
Ana. Mr. Newman pointed out the fact that methods of curing wal-
nuts had received very little attention or modification since the early
development of the walnut industry. As a result of the stimulated
interest in this subject, the California Walnut Growers' Association
appropriated a fund of $1000 to be used in studying walnut dehydra-
tion. This fund has been expended by the field department of the
Association, in cooperation with the Agricultural Experiment Station
of the University of California. The experiments herein described
may be considered a report of the expenditure of this fund, for without
this assistance in financing the investigation, the University would have
been obliged to greatly curtail its present activity on this problem.
EXTENT OF DEHYDRATION DURING 1923
In 1923 there were 22 walnut dehydraters of various types in
operation. The average crop dried per dehydrater was 72.5 tons, mak-
ing a total amount of 1593 tons. This quantity amounts in general
terms to 6.5 per cent of the total crop of walnuts in California. Of the
22 dehydraters in operation in 1923, 13 were operated for the first
time that season.
ADVANTAGES OF DEHYDRATION
Careful observation of the operation of all types of walnut dehy-
draters during the past two seasons has led to the following conclusions
regarding the advantages of dehydration as compared with sun-drying
in curing walnuts.
1. Weather Protection. — Dehydraters may be constantly operated
and thus continue the drying process day and night in any kind of
weather. Splitting of nuts by dry, hot winds or delayed drying
because of fogs or rains are both avoided, with consequent elimination
of worry, extra labor, depreciation in the quality of the nuts, and a
general financial loss to the grower during unfavorable drying weather.
2. Theft Protection. — All harvested nuts are protected from theft
by being dried and stored within the dehydrater building until
delivered to the packing house.
3. Drying Efficiency. — Accurate control of the drying air results
in the most uniform, thorough, and rapid drying consistent with the
best quality.
Bulletin 376] SUN-DRYING AND DEHYDRATION OF WALNUTS 11
4. Labor Efficiency. — Dehydraters require considerably less labor
per ton of nuts dried. This is an important consideration in the fall
of the year when the harvesting of other crops is making demands on
the labor supply in competition with walnuts.
5. Packing House Efficiency. — Dehydraters afford a steady supply
of cured nuts, enabling- packing houses to more nearly operate at their
maximum efficiency throughout the season.
The effect of some fifteen dehydraters operating in the vicinity of
the Saticoy packing house in 1923 on the daily receipts of that house
as compared with 1922 is illustrated in figure 3. Even the influence
of these few dehydraters did much to equalize the daily delivery of nuts
to the packing house with consequent reduction in the cost of packing.
The leveling effect upon the packing load of the Saticoy house
during the two seasons may be summarized as follows: The daily
packing capacity of the house is 110 tons; the maximum storage capac-
ity is 200 tons. During 1922 the house used extra crews to operate the
house 7 Sundays and 16 nights. During 1923 the house operated only
1 Sunday and 6 nights.
6. Marketing Efficiency. — Dehydration gains several days over sun-
drying; thereby facilitating early marketing which is growing in
importance as the tonnage of California walnuts increases. The peak
of demand for walnuts occurs before the period of the peak of delivery,
therefore, anything which will tend to speed up the wholesale delivery,
will stimulate the market. With ordinary harvesting methods plus the
use of a dehydrater a grower can fe 1 certain that his entire crop will
reach the packing house before the "first pool" closes. The earlier
packing house deliveries illustrated in figure 3 were largely the result
of dehydration.
PRINCIPLES OF DEHYDRATION
Dehydration may be defined as the evaporation of excess moisture
without injury to the products by carefully controlled currents of
artificially heated air. This modern method of drying has recently
received much study and development in connection with the drying
of fruits in California. The fundamental engineering principles under-
lying the construction and operation of dehydraters for walnuts are
very similar to those already published for fruits.* It was necessary,
however, to study the behavior of walnuts during dehydration in order
to determine what types of equipment and methods of operation
resulted in the maximum efficiency in drying compatible with fine
quality and economical operation.
* The Construction of Farm Dehydraters in California, by A. W. Christie and
G. B. Ridley, Journal of the American Society of Heating and Ventilating Engi-
neers, Vol. 29, No. 9. December, 1923.
12
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
The time required to evaporate the excess moisture in walnuts is
dependent on the following factors :
1. The maximum temperature at which walnuts can be safely
dehydrated.
2. The per cent of excess moisture in the nuts when harvested.
3. The volume of heated air reaching the nuts.
4. The relative humidity of the air.
/SO
/so
/60
/922
/60
MO
/40
/20
DAJLY PACK/NG CAPAC/TY
/20
/OO
/OO
80
60
60
60
*o
■
40
20
o
„ll.l
M _
1
"%
/80
/60^
/60
/923
/60
/40
HO
/20
DA/LY PACff/NG CAPAC/TY
/20
/OO
/OO
60
60
60
60
40
-
■
40
20
O
i
J
J.Ui_
.111,1 '
20
O
SEPT 2$ A/OV /S
Fig. 3. — Comparative daily delivery of nuts to Satieoy packing house.
Bulletin 376]
SUN-DRYING AND DEHYDRATION OF WALNUTS
13
The following paragraphs set forth briefly the results of numerous
tests on the dehydration of walnuts and the bearing of this informa-
tion on the construction and operation of walnut dehydraters :
TEMPERATUEE EFFECTS
The evaporation of water requires the expenditure of a definite
amount of heat (approximately 1000 British Thermal Units per pound
of water evaporated). In the case of walnut dehydraters this heat is
most conveniently and economically generated in oil-burning furnaces.
It is obvious that the higher the temperature of the air conveying this
heat from the furnace to the nuts, the more rapid will be the evapora-
tion of moisture from the nuts.
TABLE 2
Relation of Temperature op Dehydration to Quality of Walnuts
Per cent
Drying
tempera-
ture
Percentage
showing
oil on
kernel
Eating quality
water
when
harvested
After
drying
After 4
months*
32
100°F.
76%
normal
normal
37
100°F.
28%
normal
normal
46
105°F.
15%
normal
normal
38
110°F.
30%
normal
injured (stale and bitter)
30
110°F.
44%
normal
18
110°F.
20%
normal
normal.
22
110°F.
56%
normal
slightly injured
17
115°F.
34%
slightly injured
36
120°F.
60%
normal
injured (flat, slightly-
rancid)
29
130°F.
80%
pleasant but
cooked flavor
rancid (not edible)
35
140°F.
100%
unpleasant oily
flavor
rancid (not edible)
* Held in common storage.
Critical Temperature for Quality. — Most food products have a
"critical" temperature beyond which they cannot be heated without
danger of injury to their quality. The results of a series of carefully
controlled temperature tests are shown in table 2, from which it can be
seen that the "critical" temperature for walnut dehydration is in the
neighborhood of 110° F. It was also evident that the higher the per
cent of moisture in the nuts when harvested, the lower the ' ' critical ' '
temperature. The dehydrated nuts were always compared with sun-
dried nuts which came from the same respective original lots. It may
14
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
be concluded, therefore, that in no case should walnut dehydraters be
operated above 110° F., and in the case of very moist nuts from early
harvesting1, it is imperative not to exceed 105° F. Walnuts dehydrated
in 1922 were still in first-class marketable condition at the end of 18
months.
TABLE 3
Comparative Drying Data on Sun-drying and Dehydration of Walnuts
(Orange County, 1923)
Date of
harvest
Method of
drying
Per cent
water in
nuts before
curing
Per cent
of weight
lost in
drying
Per cent
of water
in cured
nuts
Per cent
of nuts
unsealed
after
curing
Drying
time,
hours
Drying
tempera-
ture
Sept. 15
Sundried
46.25
32.32
8.87
2.25
224
65°
Sept. 15
Dehydrated
46.25
36.19
5.11
0.00
46
105°
Sept. 17
Sundried
37.70
31.56
7.25
4.00
240
58°
Sept. 17
Dehydrated
37.70
33.57
5.48
0.00
36
110°
Sept. 19
Sundried
36.00
25.78
7.83
8.00
200
64°
Sept. 19
Dehydrated
36.00
30.57
5.66
0.15
24
120°
Sept. 20
Sundried
28.76
27.14
6.94
5.50
195
64°
Sept. 20
Dehydrated
28.76
27.72
7.45
0.27
18
130°
Sept. 22
Sundried
35.11
33.21
6.79
4.00
170
65°
Sept. 22
Dehydrated
35.11
27.64
6.91
0.04
14
140°
Sept. 24
Sundried
36.60
30.30
8.04
2.00
198
66°
Sept. 24
Dehydrated
36.60
30.05
9.29
0.08
30
100°
Sept. 25
Sundried
33.18
29.85
8.02
2.00
222
67°
Sept. 25
Dehydrated
33.18
27.90
8.49
0.07
42
90°
Sept. 28
Sundried
31.55
27.14
3.67
9.00
198
72°
Sept. 28
Dehydrated
31.55
25.30
7.00
0.08
32
100°
Sept. 30
Sundried
29.82
20.53
3.94
6.50
122
67°
Sept. 30
Dehydrated
29.82
20.59
4A4:
0.00
46*
110°
Oct. 2
Sundried
18.45
17.50
3.98
6.00
184
67°
Oct. 2
Dehydrated
18.45
14.24
4.74
0.00
32f
110°
Oct. 8
Sundried
21.64
16.03
3.77
11.00
120
68°
Oct. 8
Dehydrated
21.64
16.96
5.52
0.28
28
110°
Oct. 10
Sundried
16 . 72
9.69
4-04
13.00
96
71°
Oct. 10
Dehydrated
16.72
14.85
2.47
0.86
20
115°
Averages -1
Sundried
30.89
25.09
6.26
6.10
181.
66°
Dehydrated
30.89
25.46
6.09
0.15
31.
112°
* Includes 10-hour shut-down after 12 hours of drying. Fan Off.
t Includes 10-hour shut-down after 12 hours of drying. Fan On.
Because of the danger from higher temperatures, it is important
that all walnut dehydraters be adequately equipped with accurate
thermometers. The temperature of dehydraters heated by oil, gas,
steam or electricity can be automatically controlled by thermostats
which eliminate the danger of over-heating as well as the necessity for
the frequent temperature observations in manually operated plants.
BULLETIN 376] SUN-DRYING AND DEHYDRATION OF WALNUTS 15
Splitting of Nuts during Curing. — Another valuable observation
associated with quality is noted in the sixth column of table 3. The
average per cent of nuts split during- dehydration was only 0.15
per cent as compared with 6.1 per cent during sun-drying of similar
lots. In no case did the per cent of split dehydrated nuts reach 1
per cent, while in sun-drying it rose as high as 13 per cent. This is a
distinct advantage in favor of dehydration.
Relation of Temperature to Drying Time. — The comparative drying
times of sun-drying and of dehydrating nuts can be seen in the seventh
column of table 3. Sun-drying required from 4 to 10 days, averaging
7!/2 days, while dehydration required from 14 to 46 hours, averaging
31 hours. The greater speed of dehydration (about six times as fast)
is due, first, to the higher temperature obtainable, and, second, to the
maintenance of this temperature throughout the drying period. Most
of the daily evaporation of water in sun-drying occurs during a few
hours in the middle of the day, while dehydration is continuous until
completed. Cool, moist nights alternating with warm, dry days not
only delay drying but cause the splitting of nuts from alternate
expansion and contraction.
It is difficult to give an average drying time for walnuts at different
temperatures because, although the conditions of dehydration be under
exact control, the initial percentages of moisture in different lots of
nuts vary greatly. However, the following averages obtained from all
experiments made in 1922 and 1923 conclusively show the advantage
of operating a walnut dehydrater at the maximum safe temperature.
Hours required
to evaporate
Temperature 1 per cent
of moisture
90° F 2.56
100° F '. 2.17
110° F •. 1.51
The use of temperatures below the maximum safe temperature
merely results in lengthening the time of drying with consequent
decrease in the capacity of the dehydrater and increase in the cost of
drying.
Starting Dehydration at Low Temperatures. — No valid objection
was found to beginning the dehydration of walnuts at the maximum
temperature to be used. Slow "warming up" of the nuts merely
increased the time and cost of drying.
16
UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION
INTEEMITTENT OPERATION OF DEHYDRATERS
A number of dehydrater operators practice intermittent operation,
the furnace and fan being shut off for about 10 hours during every
night. It was thought that the nuts would continue drying during this
period. This practice was made the object of an exact study, the
results of which are shown in figure 4. It is evident that as soon as
the flow of heated air ceases, there is little or no further drying of the
nuts until the dehydrater is started again. In certain instances there
may be other factors, such as labor supply, which justify intermittent
operation, but from a standpoint of drying efficiency it merely reduces
the capacity and increases the cost per ton of operating the plant.
Fig. 4. — Effect of intermittent operation on dehydrating time of walnuts.
Drying temperature, 110° F.
Some operators thought that during the nightly shut-down period
a considerable amount of moisture equalization occurred between the
already dried and the still moist nuts which might be mixed together
in the bins. For the purpose of ascertaining the extent of such
equalization, a lot of thoroughly cured nuts were individually marked,
mixed and sacked with an equal weight of uncured nuts. At periodic
intervals, the respective lots were separated and weighed. The results
showed that the absorption of moisture by the cured nuts from the
BULLETIN 376] SUN-DRYING AND DEHYDRATION OF WALNUTS 17
uncured nuts was negligible and therefore of no value in obtaining
even drying.
In other words, the drying of each individual nut proceeds inde-
pendently until its moisture content reaches an equilibrium with the
moisture content of the surrounding air.
RELATION OF HUMIDITY TO DEHYDRATION
The lower the relative humidity of air the greater its moisture
absorbing power. The relative humidity of air is halved and conse-
quently its moisture holding capacity doubled for every 27° F. rise
in temperature. Hence, outside air saturated with moisture at 56° F.
would have its moisture holding capacity quadrupled when heated to
110° F. in a dehydrater. This explains why dehydraters continue
drying irrespective of external atmospheric changes.
No specific experiments were made on the effect of air of different
degrees of humidity on the drying time of walnuts, but it was observed
incidentally that a material increase in the relative humidity of the
air resulted in a slightly retarded rate of drying of the nuts. However,
there is no danger from this source since the volume of air required
to convey to the nuts the necessary heat for moisture evaporation is
several times that required to absorb this evaporated moisture. Hence,
the retardation of drying by a wholly or relatively heavily saturated
air is rarely encountered in walnut dryers, except in the case of a
natural draft dryer operated with closed vents.
RELATION OF AIR FLOW TO DEHYDRATION
Air currents serve two essential functions in any dehydrater : first,
to convey the heat required for moisture evaporation from the source
of heat to the material to be dried, and second, to absorb and carry
away the water vapor produced. The amount of evaporation in a
given time is dependent on the amount of heat reaching the nuts. The
amount of heat is in turn controlled by :
a. The temperature of the air.
b. The volume of the air.
It has already been shown that the temperature of the air must
not exceed 110° F. Consequently, any increase in the speed of drying
must be brought about by increasing the volume of air and therefore
the amount of heat reaching the nuts in a given time.
Air flow in dehydraters is secured in one of two ways:
a. Natural draft produced by the expansion of heated air.
b. Forced draft produced by power-driven fans.
18
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
The latter method is used in most modern dehydraters because
natural drafts are usually inadequate in volume and difficult to
properly direct and distribute. The effect of increased air flow on
reducing the drying time of walnuts can be seen in table 4. In walnut
tray dehydraters, velocities in excess of 500 feet per minute over the
trays do not warrant the increased expenditure for such air flow. On
the other hand, air velocities much below 500 feet per minute greatly
increase the time and cost of drying.
TABLE 4
Eelation of Air Velocity to Drying Time of Walnuts
Method of drying
Average
temperature
Air
velocity
(ft. per min.)
Drying
time
(hours)
Per cent
water
lost
Sun-dried
72
100
100
100
250
500
1000
122
40
32
28
27
Dehydrater
24
Dehydrater
25
Dehydrater
21
Dehydrater
100
100
Nat. Draft
500
33
16
9
Dehydrater
9
Most walnut dryers built so far are imperfect because of inadequate
and uneven air flow. Only by the use of fans of sufficient power can
the most rapid and uniform drying be obtained. Dependence on
natural draft or the use of fans of insufficient capacity is false economy.
EELATION OF DEPTH OF WALNUTS TO AIE FLOW
Since the bin type of dryer seems to be the most generally satisfac-
tory for walnuts, an experiment was made to determine the effect of
various depths of walnuts on the flow of air through them. From the.
graphic presentation of the results in figure 5, it can be seen that the
first 6 to 12 inches of walnuts practically determines the volume of air
which a given fan can force through the nuts. Increasing the depth
of nuts does not alter the free area for air passage but merely decreases
the air flow slowly in proportion to the increased frictional resistance
to its passage. In bin dryers the heated air should be delivered from
the fan to the bottoms or sides of the bins by the largest, shortest, and
straightest flues which can be used. The air should enter at a central
point from which it can be equally distributed to all parts of the bin.
This can be accomplished by causing the air to enter at the center of
the bottom or side of the bin from where it can be evenly distributed
over the entire bin by proper arrangement of baffles.
BULLETIN 376] SUN-DRYING AND DEHYDRATION OF WALNUTS
19
2300
Ifc 16* Z* 30 36 H-Z H8 5+ 60
Depth of Walnuts (inches)
Fig. 5. — Effect of depth of walnuts on air velocity.
20 UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION
YIELDS AND STOEAGE OP DEHYDRATED WALNUTS
Since the curing of walnuts consists of the evaporation of excess
moisture, there should be no difference in yields by sun-drying or
dehydration, provided the nuts are reduced to the same degree of
dryness in both cases. From table 3 it can be seen that, although there
were occasional differences in the losses in weight by the two methods
of drying, these differences were not consistently in favor of either
method and that the seasonal averages were almost identical.
Figure 6 shows the average changes in weight during storage for
the dehydrated and for the sun-dried nuts referred to in table 3. The
unusually dry winter of 1923-24 in southern California caused both
lots to lose over 2 per cent of their weight in storage, but during a
period of relatively high humidity caused by spring rains the nuts
regained their original weight, showing that they had been dried to
the proper degree for normal winter storage. The dehydrated nuts
had evidently been cured a little more thoroughly, as shown by the
average moisture contents given at the bottom of column 5, table 3.
This was confirmed by their slightly smaller change in weight during
storage.
SEPT 2 7 OCT. 15 NOV. 15 0€C. // JA/v. /J
DA TE WE'GHED
Fig. 6. — Changes in weights of walnuts during storage (Riverside, 1923-24).
BULLETIN 376] SUN-DRYING AND DEHYDRATION OF WALNUTS 21
TYPES OF WALNUT DEHYDRATERS
It is not the purpose of this publication to present complete plans
and specifications for dehydrater construction. However, a brief
description of the more common types of walnut dehydraters, with
their relative advantages and disadvantages, should be of interest to
prospective purchasers.
NATURAL DEAFT DEYEES
Probably the simplest form of dryer consists in closing drying
houses, such as the Sidwell Dryer described on page 7, with solid
walls and placing one or more small furnaces on the ground under the
bottom floor. This permits such a drying house to continue drying in
cold or damp weather, because the outer air drawn in through openings
near the ground is heated by passing over the furnaces, rises through
the floors of nuts and eventually escapes through a ventilator in the
roof, carrying with it some moisture evaporated from the nuts.
In another type of natural draft dryer a series of vertical screen
walls, some 6 inches apart, is substituted for the screen floors. The
space between each alternate pair of walls is filled from above with
nuts, the alternate open spaces being reserved for the upward passage
of heated air.
These and similar modifications of natural draft dryers have two
distinct advantages over sun-drying in that they reduce labor in
handling the nuts and continue drying in all kinds of weather. From
the standpoint of efficient dehydration, however, such dryers are
imperfect. They occupy more space than bin type dehydraters of
equal capacity. The air flow is only sufficient for relatively slow
drying. Since natural air currents follow paths of least resistance, it
is very difficult to distribute the air and heat so as to insure even dry-
ing, especially when the dryer is only partly filled. The temperature
in natural draft dryers is subject to wide fluctuations which cannot
be regulated without close watching.
AIE BLAST DEHYDEATEES
The use of fans in dehydraters to furnish a strong, constant flow
of heated air, the temperature and distribution of which are under
rather exact control, has now become almost universal. Although
there are numerous modifications in this class, the tunnel type using
trays and the bin type are of greatest interest because both have been
successfully used on walnuts for several years.
22
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
Bin Dehydraters are a class developed for the drying of walnuts
only. They consist of three essential parts: one or more bins for
holding the nuts, a furnace or other device for generating heat, and a
fan for conveying heated air from the furnace to the bins.
One type consists of bins about 10 feet square, the bottoms of which
are made of slats or screen and in the form of inverted pyramids. In
another type (illustrated in figs. 7, 8, and 9) rectangular bins, usually
6 feet by 16 feet, are built on about a 30 degree slope, with a false,
slatted floor forming an air chamber some 6 inches deep beneath the
nuts. The capacity of a bin dryer may be determined by multiplying
the cubical contents by 35 pounds of cured nuts per cubic foot.
Fig. 7. — Full and empty bins and sacking device. Mahoney bin dryer.
A blast of heated air is introduced into the bottom of each bin
through flues leading from a fan, one fan and furnace usually supply-
ing a battery of four bins. The bins are filled with nuts from above to
a depth of 2 to 3 feet by means of a simple elevator and conveyor,
giving a maximum capacity per bin of 2y2 tons of nuts.
In both of these types the air heating equipment is located close by.
This usually consists of either a standard oil burning furnace, such as
is used for heating buildings, or a simpler heating chamber utilizing
direct heat from electricity or gas. Outside air is drawn through the
heating chamber by a fan and delivered through flues to the bottoms
of the several bins. It then rises through the mass of nuts and
eventually escapes into the outer air, carrying with it the moisture
which the heat has caused to evaporate from the nuts.
Bulletin 376]
SUN-DRYING AND DEHYDRATION OF WALNUTS
23
The best type of fan for this purpose is that known as the Multi-
vane, which delivers the maximum volume of air for the horsepower
expended and operates successfully against considerable back pressure
caused by resistance to the flow of the air through the bins of nuts.
The time required in such dryers varies according to the moisture
content of the nuts and the operating temperature. Constant operation
at 100° F. to 110° F. will complete drying in 24 to 48 hours. Proper
Fig.
-Elevator and conveyor for filling a Mahoney bin dryer.
construction and arrangement of the bins and air ducts will result in
uniform distribution of heated air throughout the bin and consequently
in even drying.
When the nuts are dry they are either drawn by gravity directly
into sacks from the bottom of each bin or discharged into a common
conveyor which carries the nuts to a central point for sacking.
24
UNIVERSITY OF CALIFORNIA — EXPERIMENT STATION
These bin dryers are generally considered the most satisfactory
type for walnuts yet developed. They provide the maximum daily
capacity for the investment involved and the space occupied. They
are susceptible of exact yet simple regulation of the temperature and
distribution of air and consequently permit the most rapid and
thorough drying consistent with quality. They are most economical
of labor, one man being easily able to load, dry, unload and sack 5 tons
of nuts daily.
The dryers of this type so far constructed have not realized their
greatest possible efficiency because of the use of fans and motors of
inadequate size. The lower the air flow the slower and more uneven
the drying. It is also probable that recirculation of the warm exhaust
air, as practiced in all modern fruit dehydraters, would result in
greater fuel efficiency.
Fig. 9. — Furnace, fan and flues for distributing heated air to a four-bin
Mahoney dryer.
Tray Dehydraters. — Tray dehydraters for fruits are now in exten-
sive use in California as well as in the Pacific Northwest. The soft
texture of all fruits except apples necessitates the use of trays in dry-
ing. There are many types of tray dehydraters in use, but the type con-
sidered most generally satisfactory is known as the Tunnel dehydrater,
consisting of a long chamber 6 or more feet wide by 7 feet high. The
trays are entered at the cooler end on slides or cars and moved pro-
gressively toward the hotter end from which they are removed when
BULLETIN 376] SUN-DRYING AND DEHYDRATION OF WALNUTS 25
the contents are dried. Some of the small dehydraters depend on a
natural draft of heated air, but most of the modern installations
include a furnace and fan located in an auxiliary chamber alongside,
above or below the drying" tunnel in such a way that a powerful, con-
trolled stream of heated air is blown between the trays of material in
the tunnel and either discharged or recirculated from the exhaust end
of the tunnel.
This type of dehydrater has been successfully used on walnuts in
both Oregon and California. It gives the most rapid and uniform
drying of all types and is most economical of fuel. However, the first
cost is greater for its capacity than that of bin dryers and the extra
labor required in loading and unloading the trays makes the cost of
drying walnuts considerably greater. The capacity of a tray dehy-
drater may be determined by multiplying* the total square feet of tray
surface by 2 pounds of cured nuts per square foot for trays filled
one nut deep. Except for growers who desire to dry other fruits as
well, tray dehydraters are not advisable. Since the economic dehy-
dration of all fruits necessitates the use of higher temperatures than
are permissible for walnuts, fruits and walnuts cannot be dried
simultaneously.
MECHANICAL MOVEMENT OF NUTS DURING DRYING
Dehydraters involving mechanical devices for the constant or inter-
mittent movement of nuts during drying are unnecessarily complicated
and expensive to build and operate. It is sounder in principle to bring
the warm air to the nuts than to attempt to bring the nuts to the air.
COSTS OF DEHYDRATION
The advantages of walnut dehydration listed on page 10 would be
of little interest to growers if such advantages could only be obtained
at greatly increased costs. The use of walnut dehydraters has not yet
become sufficiently general to permit of the gathering of extensive data
on the cost of dehydration. However, the costs of operation presented
in table 5 are considered typical of present practice.
It can be seen that although the fixed charges per ton for the bin
dryer are greater than for sun-drying, because of the greater invest-
ment, the operating costs are sufficiently lower, because of labor saving,
to make the total cost about the same. The moderate cost of power
and fuel is easily counterbalanced by the saving in labor. It is reason-
26
UNIVERSITY OF CALIFORNIA EXPERIMENT STATION
able to expect that, as the construction and operation of walnut dehy-
draters is perfected, their cost of operation will decrease to an amount
less than that of sun-drying, as already realized in the case of modern
prune dehydraters,
TABLE 5
Comparative Costs per Ton of Sun-drying and Dehydrating Walnuts in 1923
Sun drying
Bin dryer
Tray Dryer
Labor
$3.77
$1.61
1.04
.23
$3 13*
Fuel
2.12f
92
Power
Total Operating Cost
$3.77
1.71
$5.48
$2.88
2.75
$5.63
$6 17
Fixed Charges!
2 00
Total all costs
$8 17
* Includes hulling and washing.
t Electric heat; equivalent to 14 cents if oil used.
j Interest and depreciation at 11 per cent.
Walnut dehydraters are still in a state of development, but they
have already sufficiently established their desirability to warrant care-
ful consideration by all walnut growers. Their advantages are
obtainable at no greater final cost than that of sun-drying. Their only
disadvantage is the greater initial cost of the equipment.
The cost of a completely equipped and housed walnut dehydrater
will vary considerably with the type, size, materials of construction,
etc., but for an efficient dehydrater of medium size will approximate
$500 to $600 per ton daily capacity.
STATION PUBLICATIONS AVAILABLE FOE FREE DISTRIBUTION
BULLETINS
No. No.
253. Irrigation and Soil Conditions in the 339.
Sierra Nevada Foothills, California.
261. Melaxuma of the Walnut, "Juglans 341.
regia." 343.
262. Citrus Diseases of Florida and Cuba 344.
Compared with those of California.
263. Size Grades for Ripe Olives. 346.
268. Growing and Grafting Olive Seedlings. 347.
273. Preliminary Report on Kearney Vine-
yard Experimental Drain. 348.
275. The Cultivation of Belladonna in Cali- 349.
fornia.
276. The Pomegranate. 350.
277. Sudan Grass. 351.
278. Grain Sorghums. 352.
279. Irrigation of Rice in California.
280. Irrigation of Alfalfa in the Sacramento 353.
Valley. 354.
283. The Olive Insects of California. 355.
285. The Milk Goat in California. 357.
286. Commercial Fertilizers.
287. Vineerar from Waste Fruits.
294. Bean Culture in California. 358.
298. Seedless Raisin Grapes.
304. A Study of the Effects of Freezes on 359.
Citrus in California. 360.
808. I. Fumigation with Liquid Hydrocyanic
Acid. II. Physical and Chemical Prop- 361.
erties of Liquid Hydrocyanic Acid.
312. Mariout Barley. 362.
317. Selections of Stocks in Citrus Propa- 363.
gation.
319. Caprifigs and Caprification. 364.
321. Commercial Production of Grape Syrup.
324. Storage of Perishable Fruit at Freezing 366.
Temperatures.
325. Rice Irrigation Measurements and Ex- 367.
periments in Sacramento Valley,
1914-1919. 368.
328. Prune Growing in California.
331. Phylloxera-Resistant Stocks. 369.
334. Preliminary Volume Tables for Second- 370.
Growth Redwood. 371.
335. Cocoanut Meal as a Feed for Dairy
Cows and Other Livestock.
336. The Preparation of Nicotine Dust as
an Insecticide.
The Relative Cost of Making Logs from
Small and Large Timber.
Studies on Irrigation of Citrus Groves.
Cheese Pests and Their Control.
Cold Storage as an Aid to the Market-
ing of Plums.
Almond Pollination.
The Control of Red Spiders in Decidu-
ous Orchards.
Pruning Young Olive Trees.
A Study of Sidedraft and Tractor
Hitches.
Agriculture in Cut-over Redwood Lands.
California State Dairy Cow Competition.
Further Experiments in Plum Pollina-
tion.
Bovine Infectious Abortion.
Results of Rice Experiments in 1922.
The Peach Twig Borer.
A Self-mixing Dusting Machine
Applying Dry Insecticides
Fungicides.
Black Measles, Water Berries,
Related Vine Troubles.
Fruit Beverage Investigations.
Gum Diseases of Citrus Trees in Cali-
fornia.
Preliminary Yield Tables for Second
Growth Redwood.
Dust and the Tractor Engine.
The Pruning of Citrus Trees in Cali-
fornia.
Fungicidal Dusts for the Control of
Bunt.
Turkish Tobacco Culture, Curing and
Marketing.
Methods of Harvesting and Irrigation
in Relation to Mouldy Walnuts.
Bacterial Decomposition of Olives dur-
ing Pickling.
Comparison of Woods for Butter Boxes.
Browning of Yellow Newtown Apples.
The Relative Cost of Yarding Small
and Large Timber.
for
and
and
CIRCULARS
No. No.
70. Observations on the Status of Corn 161.
Growing in California. 164.
87. Alfalfa. 165.
111. The Use of Lime and Gypsum on Cali-
fornia Soils. 166.
113. Correspondence Courses in Agriculture. 167.
117. The Selection and Cost of a Small 170.
Pumping Plant.
127. House Fumigation. 172.
129. The Control of Citrus Insects. 173.
136. Melilotua indica as a Green-Manure
Crop for California. 174.
144. Oidium or Powdery Mildew of the Vine. 175.
151. Feeding and Management of Hogs.
152. Some Observations on the Bulk Hand- 178.
ling of Grain in California. 179.
153. Announcement of the California State
Dairy Cow Competition, 1916-18. 182.
154. Irrigation Practice in Growing Small
Fruit in California. 184.
155. Bovine Tuberculosis. 188.
157. Control of the Pear Scab. 190.
158. Home and Farm Canning. 193.
160. Lettuce Growing in California. 198.
Potatoes in California.
Small Fruit Culture in California.
Fundamentals of Sugar Beet Culture
under California Conditions.
The County Farm Bureau.
Feeding Stuffs of Minor Importance.
Fertilizing California Soils for the 1918
Crop.
Wheat Culture.
The Construction of the Wood-Hoop
Silo.
Farm Drainage Methods.
Progress Report on the Marketing and
Distribution of Milk.
The Packing of Apples in California.
Factors of Importance in Producing
Milk of Low Bacterial Count.
Extending the Area of Irrigated Wheat
in California for 1918.
A Flock of Sheep on the Farm.
Lambing Sheds.
Agriculture Clubs in California.
A Study of Farm Labor in California.
Syrup from Sweet Sorghum.
CIRCULARS — Continued
No.
199.
201.
202.
203.
205.
206.
208.
209.
210.
212.
214.
215.
217.
218.
219.
228.
230.
231.
232.
233.
234.
235.
236.
237.
238.
239.
240.
241.
Onion Growing in California.
Helpful Hints to Hog Raisers.
County Organizations for Rural Fire
Control.
Peat as a Manure Substitute.
Blackleg.
Jack Cheese.
Summary of the Annual Reports of the
Farm Advisors of California.
The Function of the Farm Bureau.
Suggestions to the Settler in California.
Salvaging Rain-Damaged Prunes.
Seed Treatment for the Prevention of
Cereal Smuts.
Feeding Dairy Cows in California.
Methods for Marketing Vegetables in
California.
Advanced Registry Testing of Dairy
Cows.
The Present Status of Alkali.
Vineyard Irrigation in Arid Climates.
Testing Milk, Cream, and Skim Milk
for Butterfat.
The Home Vineyard.
Harvesting and Handling California
Cherries for Eastern Shipment.
Artificial Incubation.
Winter Injury to Young "Walnut Trees
during 1921-22.
Soil Analysis and Soil and Plant Inter-
relations.
The Common Hawks and Owls of Cali-
fornia from the Standpoint of the
Rancher.
Directions for the Tanning and Dress-
ing of Furs.
The Apricot in California.
Harvesting and Handling Apricots and
Plums for Eastern Shipment.
Harvesting and Handling Pears for
Eastern Shipment.
Harvesting and Handling Peaches for
Eastern Shipment.
No.
243.
244.
245.
247.
248.
249.
250.
251.
252.
253.
254.
255.
256.
257.
258.
259.
260.
261.
262.
263.
264.
265.
266.
267.
268.
269.
270.
271.
Marmalade Juice and Jelly Juice from
Citrus Fruits.
Central Wire Bracing for Fruit Trees.
Vine Pruning Systems.
Colonization and Rural Development.
Some Common Errors in Vine Pruning
and Their Remedies.
Replacing Missing Vines.
Measurement of Irrigation Water on
the Farm.
Recommendations Concerning the Com-
mon Diseases and Parasites of
Poultry in California.
Supports for Vines.
Vineyard Plans.
The Use of Artificial Light to Increase
Winter Egg Production.
Leguminous Plants as Organic Fertil-
izer in California Agriculture.
The Control of Wild Morning Glory.
The Small-Seeded Horse Bean.
Thinning Deciduous Fruits.
Pear By-products.
A Selected List of References Relating
to Irrigation in California.
Sewing Grain Sacks.
Cabbage Growing in California.
Tomato Production in California.
Preliminary Essentials to Bovine Tuber-
culosis Control.
Plant Disease and Pest Control.
Analyzing the Citrus Orchard by Means
of Simple Tree Records.
The Tendency of Tractors to Rise in
Front: Causes and Remedies.
Inexpensive Labor-saving Poultry Ap-
pliances.
An Orchard Brush Burner.
A Farm Septic Tank.
Brooding Chicks Artificially.
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